The seven-segment numeric display unit, as disclosed for example in U.S. Pat. No. 3,943,500, has become increasingly popular in recent years. Such units employ a seven bar readout which in any one of ten combinations can simulate numbers 1 through 9 or 0. They are commonly used with electronics in providing an ongoing display of numerical data in response to a changing electronic information input. A commercial form of these units is available from Ferranti-Packard, Ltd. of Toronto, Ontario, Canada, in which the individual bar or segment is rotatably supported at a housing window. Each bar has a non-display side finished to blend or match with the viewing face of the housing while the opposite side is finished in a light reflective material to contrast with and render it visually distinctive from the viewing face of the housing. An electromagnetic coil operatively associated with each segment will, when energized, rotate the segment between the non-display and display positions. A typical application for such displays is on electronically controlled gasoline dispensers for providing volume, price and unit price information transacted with the dispenser.
While the seven-segment display has proven itself over extended periods of use, they are susceptible to partial failures by inoperation of one or more of the individual segments. Since any segment failure results in an inaccurate display of the information sought to be provided, it has become increasingly common to employ a feedback detection circuit for identifying display inaccuracies as disclosed, for example, in mentioned U.S. Pat. No. 3,943,500. One approach to failure detection has been to utilize individual current sensors in series with the solid state drivers that operate each of the segments in order to measure the current level achieved by the sensors. While believed to function accurately, the multiplicity of the individual sensor approach has required both an excessive power supply and excessively large solid state switch devices for switching the encountered current levels. These factors have therefore rendered that approach somewhat objectionable, but despite recognition of the problem a ready solution has not heretofore been known.